1. Field of the Invention
The invention relates to an infrared ray image pickup device, and more particularly to an improvement of a thermal infrared ray image pickup device having thermoelectric transducers that convert heat to an electric signal and two dimensional arrays of pixels.
2. Description of the Related Art
The infrared ray image pickup devices have been widely used in the fields of crime prevention, observation, guidance, medical treatment and industrial measurements and others. A requirement for a real time image pickup of temperature distributions of objects or articles has been on the increase. Under these circumstances, a requirement for an improvement of the two-dimensional array infrared ray image pickup devices in view of a reduction of a cost thereof has also been on the increase.
Various types of infrared ray image pickup devices have been known. A first conventional type infrared ray image pickup device has a two story structure wherein the first story includes a scanning circuit for scanning signals for each pixel involved in the infrared ray image pickup device and the second story includes thermoelectric transducers for converting heat to electric signals. Further noise reduction circuits are provided in each pixel.
Such a conventional infrared ray image pickup device is illustrated in FIGS. 1A and 1B. The first type conventional infrared ray image pickup device comprises two-dimensional arrays of pixels that are provided on a semiconductor substrate 19. Scanning circuits 20 are provided for each pixel on the semiconductor substrate 19. Cavities 22 are provided for each pixel on an oxide film 21 that is provided on the scanning circuits 20 so that the scanning circuits and the cavities 22 are separated through the oxide film 21. Diaphragms 24 are provided for each pixels over the cavities 22. The diaphragm 24 comprises a thin oxide film. Thermoelectric transducers 23 are provided for each pixel within the diaphragms 24 so that each of the thermoelectric transducer 23 is sandwiched between upper and bottom portions of each of the diaphragms 24. The thermoelectric transducer 23 comprises an element for converting heat to electric signals, for example, a thermocouple. In each pixel, the thermoelectric transducer 23 is electrically connected through a metal wire 25 made of aluminum to the scanning circuit 20.
An incidence of infrared rays on the top surface of the diaphragm 24 causes a variation in temperature of the top portion of the diaphragm 24 so that the thermoelectric transducer 23 senses the temperature variation to convert it to electric signals that are subsequently transferred to the scanning circuit 20 through the metal wiring 25. The scanning circuit 20 receives the electric signals to integrates the received electric signals for noise reduction. Further, the scanning circuit 20 conducts the scanning of the electric signals for each pixel for subsequent transfer of the scanned signals to the exterior of the device. The above conventional infrared ray image pickup device is disclosed in the Japanese Patent Application No. 5-183116.
The first type of the conventional infrared ray image pickup device has a problem with the diaphragm 24 that may permit a heat caused by the infrared ray incidence on the surface of the diaphragm to be prevented by a thermal conduction through the diaphragm. This may restrict an improvement in a sensitivity of the infrared ray image pickup device. As a modification, removal of a part of the diaphragm to prevent the thermal conduction may result in a reduction of an infrared ray receiving area in the infrared ray image pickup device. The reduction of the infrared ray receiving area may of course restrict the required improvement in the sensitivity of the infrared ray image pickup device.
A second type of the conventional infrared ray image pickup device is illustrated in FIG. 2. The second type of the conventional infrared ray image pickup device has a thermoelectric transducer 28 in the form of a beam 29 extending over a V-shaped groove formed in an upper portion of a semiconductor substrate 26. A surface of the V-shaped groove of the semiconductor substrate 26 is coated with an infrared ray reflection film 27 that serves as a converging mirror to converge the reflected infrared ray toward the beam 29 of the thermoelectric transducer 28. The structure of the thermoelectric transducer 28 in the form of the beam 29 extending over the V-shaped groove may be formed by an isotropic etching technique in forming the V-shaped groove after the thermoelectric transducer 28 is formed to overlie the surface of the semiconductor substrate 26. The above conventional infrared ray image pickup device is disclosed in the Japanese Laid-open Patent Application No. 4-132271.
The second type of conventional infrared ray image pickup device has a problem with a difficulty in forming scanning circuits on the semiconductor substrate 26 having the V-shaped groove, thereby resulting in a difficulty in obtaining a high density integration of the two dimensional arrays of the pixels.
A third type of the conventional infrared ray image pickup device is illustrated in FIG. 3. The third type of conventional infrared ray image pickup device has micro-lenses, each of which may serve as a converging lens to converge infrared rays. The micro-lenses are provided on a sensor substrate 30. The micro-lenses include cavities 33 filled with vacuum wherein the vacuum cavities 33 are positioned over the top of the sensor substrate 30. Each of the vacuum cavities 33 accommodates a thermoelectric transducer 31 that is provided on the top of the sensor substrate 30. The above conventional infrared ray image pickup device is disclosed in the Japanese Laid-open Patent Application No. 3-263001.
The third type of the conventional infrared ray image pickup device has the following two problems. The micro-lenses 32 are formed on a lens substrate. The lens substrate is required to have a thickness of at least about 100 micrometers. Such thickness of the lens substrate may cause a first problem as described below and illustrated in FIGS. 4A and 4B. Obstacle and vertical incidences of the infrared rays have a variation of a focal point on the thermoelectric transducer 31 in a lateral direction parallel to the surface of the layers. Particularly, the variation of the focal point in the lateral direction is a serious problem as the pixel size comes into the same size as the thickness of the lens substrate. An infrared ray passing through a peripheral portion of the lens comes into an obstacle incidence on the thermoelectric transducer. The infrared ray having the obstacle incidence has the focal point varied from the thermoelectric transducer in the lateral direction. Namely, the infrared ray having the obstacle incidence is irradiated on a point away from the thermoelectric transducer, for which reason the thermoelectric transducer is incapable of sensing the infrared ray of the obstacle incidence so that the thermoelectric transducer is able to sense only an infrared ray passing through the lens in the vicinity of the optical axis thereof. This results in a lowering of a sensitivity of the infrared ray image pickup device.
A second problem with the third type of the conventional infrared ray image pickup device is in a difficulty in providing a beam structure of pixels for prevention of a heat conduction. It is difficult to permit the beam structure to have a sufficient mechanical strength to support the micro lens. In a high density integration of the pixels, an alignment of the micro-lenses on the lens substrate may expose the beam structure to danger of destruction. Consequently, it is difficult to provide the beam structure to the infrared ray image pickup device for prevention of the lowering of the sensitivity thereof.